1. The problem statement, all variables and given/known data Light with wavelength 100nm is incident perpendicularly from air on a film 0.500μm thick and with refractive index 2.5. Part of the light is reflected from the first surface of the film, and part enters the film and is reflected back at the second surface, where the film is again in contact with air. What is the phase difference? 2. Relevant equations phase difference = (2pi*deltaL)/lambda 3. The attempt at a solution found the angle then added the vectors, used cosine rule and got an answer similar to it.